System and method for determining the polarity of an electrostatic event

ABSTRACT

A reusable miniaturized detector that utilizes magneto-optic elements to detect the occurrence of an electrostatic discharge during the manufacture or handling of electrostatic discharge sensitive electronic components and circuit boards. The device may also be used to determine the polarity and magnitude of the electrostatic discharge. The device may be manually or automatically read, either by removing the device from the environment being monitored or continuously monitoring in situ. The device can also be configured to provide protection to some electrostatic discharge events which could damage sensitive components being monitored.

RELATED APPLICATIONS AND PRIORITY DOCUMENTS

This application is a divisional application of Ser. No. 08/714,411,filed Sep. 16, 1996, now U.S. Pat. No. 6,172,496 issued on Jan. 9, 2001.

This application claims the benefit of U.S. Provisional Application Ser.No. 60/003,894, filed Sep. 18, 1995, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to new and improved devices fordetecting electrostatic discharge (ESD) events occurring in electroniccomponents and electronic assemblies and, more particularly, to reusabledetectors which can be positioned directly on or closely adjacent tominiature electronic components and printed circuit board assemblies todetect the occurrence, polarity and approximate magnitude of an ESDevent. The invention also relates to devices to protect electroniccomponents and circuits from static discharge, in that the invention canbe connected to an electrostatic discharge sensitive device so that ESDevents above a predetermined magnitude would result in the destructionof the apparatus of the invention, rather than the ESD sensitive device.

2. Description of the Related Art

Static charges may accumulate on the surfaces of non-grounded conductorsand non-conductive surfaces such as most plastics and textiles. Thesecharges will usually remain on the surfaces because there is no path toground. When a non-grounded conductor comes close to a grounded plane, aspark of high voltage, and potentially destructive low current, will“leap” from a point on the non-grounded conductor to the groundedobject, causing an electrostatic discharge (ESD) event. When a chargednon-conductor comes close to a conductive object, the charge can beinduced onto the conductor, which can then rapidly discharge to otherconductors.

Electrostatic discharge occurs in many industrial situations, such asmanufacturing and assembly processes, electronic testing processes andthe like. For example, in the manufacture of semiconductor devices,electrostatic charges may build up and become discharged during varioushuman or machine handling operations wherein semiconductor wafers areprocessed, tested and packaged. The amount of electrostatic chargeaccumulated and discharged during handling of work pieces can besufficiently high to cause a significant number of component failures,reducing the yield of the various manufacturing, testing and packagingoperations and thereby increasing the overall cost of the device.

Static electricity can create a wide variety of problems for electronicmanufacturers. An ESD event can cause a rapid electron movement throughthe microscopic conductive paths within a device and generate a heatspike which can cause damage to the gates or other insulating parts ofthe electronic device. If the discharge is large enough, a portion ofthe device will be destroyed and the defect will be found duringtesting. While high levels of electrostatic discharge will result in theimmediate destruction of the device, which can be readily discoveredduring subsequent testing, low level electrostatic discharge may causelatent damage to the device, which may not be detected during initialtesting. This latent damage may later result in reduced performanceand/or premature product failure.

At present, efforts have been directed toward prevention of ESD eventsduring manufacturing, since there are few known methods to monitoractual events. Knowing where, how large, and when an event occurs isuseful in evaluating ESD induced failures so that appropriate preventivemeasures can be taken to eliminate the source of the ESD event.

Improvements in the manufacture of semiconductor devices have resultedin devices having vastly increased circuit density, reduced activeelement size and reduced conductor widths. These improvements haveincreased the overall performance of the devices, but havesimultaneously increased the susceptibility of the devices to damagefrom electrostatic discharge. As a result, electronic devices arepotentially susceptible to damage from discharge events as low as 50volts. Thus, semiconductive devices in routine manufacture and use todayare more susceptible than humans, who normally can not feel anelectrostatic discharge of less than approximately 3500 volts.

A variety of instruments have been designed and developed to measureelectrostatic phenomena in semiconductor device assembly areas. Some ofthese devices are connected directly to the circuit boards, while otherinstruments have antennas or other sensors that detect electromagneticradiation resulting from an electrostatic discharge. In general, theseinstruments suffer from one or more disadvantages that limit theiracceptance and use in the electronic industry, and similar industries.These disadvantages include being too large or expensive, difficult tomonitor in real time, and non-reusable, or some combination of thesedrawbacks. These prior art instruments also generally fail to providesufficient information to assist in the detection of devices that aredamaged or destroyed, including information leading to the detection andelimination of the incipient environmental causes of the ESD events.

One prior art device used to detect ESD events utilizes a silicon FieldEffect Transistor (FET) which is destroyed in the process. This deviceis monitored by first removing it from the circuit board and theninserting it into an external reader. This device has inherentdisadvantages which include its inability to be reused, the requirementthat it be removed from the circuit to be tested, its inability tomeasure polarity, and its limited range of one ESD sensitivity level perdevice.

Other prior art detection devices are known which utilize a liquidcrystal display as an indicator and has a clip lead which can beconnected to the particular position of interest, e.g., input to an ESDsensitive device. This particular device has a built-in antenna whichsenses the ESD event and includes hardware for mounting and protectingthe device while in use. This device also has inherent disadvantageswhich include the large size of the unit, its low operating/storagetemperature range, its ability to detect only one transient voltage, andthe high cost in manufacturing the unit. Additional disadvantagesinclude incompatibility with automatic insertion equipment and theinability of the device to measure polarity.

What has been needed, and heretofore unavailable, is a reliable, lowcost, rugged, miniature, reusable device for accurately and economicallydetecting the occurrence, polarity and magnitude of electrostaticdischarge events, including relatively low level events, which can alsoprovide protection to semiconductive devices from large electrostaticdischarges. Such a device should be capable of memory retention, so thatthe occurrence of an ESD event can be detected anytime after it happens.The present invention satisfies these and other needs.

SUMMARY OF THE INVENTION

The present invention is directed to a reusable, miniaturizedmagneto-optic device that detects the current of an electrostaticdischarge which may occur during the manufacture, handling or use ofelectrostatic discharge sensitive components and circuit boards. Thepresent invention also may be used to determine the polarity andmagnitude of an electrostatic discharge event. A device made inaccordance with the present invention may be manually or automaticallyread, either by removing the device from the environment being monitoredor monitoring the device in situ. The present invention can also be usedas a protection device which can be connected to an electrostaticdischarge sensitive component to protect it from electrostatic dischargeevents above a predetermined magnitude.

The electrostatic discharge event detector of the present inventionemploys the magneto-optic Faraday effect to detect electricaltransients. The Faraday effect is a scientific principle which causesthe plane of polarization of a polarized beam of light passing through atransparent substance exhibiting the Faraday effect to rotate from theplane of polarization of the incident light by an amount proportional tothe magnetic field passing through the substance parallel to the opticalaxis of the beam of light. Magneto-optic materials exhibiting theFaraday effect are electrically addressable and change or “switch” thedirection of magnetization of a magnetic material formed into individualelements, or pixels, through electrical conductors or drive lines thatestablish a magnetic field having a different direction of magnetizationto the initial state. When a magnetic field is established havingstrength equal to or greater than a predetermined value, the reversal ofthe direction of magnetization, or switching, occurs.

The electrostatic discharge event detector of the present inventionincludes a conductor and at least one magneto-optic pixel locatedadjacent to the conductor which has a first magnetic state and which iscapable of switching to a second magnetic state in response to amagnetic field having a field strength that exceeds a predeterminedfield strength at the location of the magneto-optic pixel. Thismagneto-optic pixel is capable of switching from its first magneticstate to its second magnetic state when an electrostatic dischargeinduces a current to flow through the conductor which is of sufficientstrength to generate a magnetic field around the conductor that exceedsthe predetermined field strength. Afterwards, the magneto-optic pixelcan be observed to determine whether it has been switched from its firstmagnetic state to the second magnetic state which would indicate that anelectrostatic discharge event has been experienced and detected by themagneto-optic pixel.

In a presently preferred embodiment, the electrostatic discharge eventdetector of the present invention also may include a secondmagneto-optic pixel having a first magnetic state and having thecapability of switching to a second magnetic state in response to amagnetic field. This second magneto-optic pixel would be located next tothe conductor on a side opposite from the first magneto-optic pixel. Asa result, the detector would utilize a pair of magneto-optic pixelslocated on opposite sides of the conductor which can be used todetermine the polarity, or direction, of the current flow of theelectrostatic discharge event. Such a determination may prove importantin analyzing the electrostatic discharge event, as it can indicate onwhich surface the electrostatic charge was accumulating.

A further embodiment of the present invention provides information aboutthe magnitude of the electrostatic discharge event, provided it exceedsa minimum magnitude. In this particular embodiment, the ESD detectorincludes a set of pixels which are arranged in an array around aconductor. Each magneto-optic pixel is separated from the conductor by adistance which is different from any other pixel. Since the strength ofthe magnetic field surrounding the conductor is inversely proportionalto the distance from the conductor where the magnetic field strength ismeasured, the different pixels will have altered magnetic statesdependent upon the strength of the magnetic field at each location.Since the strength of the magnetic field is related to the magnitude ofthe current flowing through the conductor induced by the electrostaticdischarge event, by observing which pixels have altered magnetic states,one can determine the voltage of the ESD event. Additionally, the pixelscan be placed in pairs on opposite sides of the conductor to determinepolarity.

The readout of the electrostatic discharge event detector can beachieved either manually by observing the reflected light through apolarizing microscope or automatically using a version of a patternrecognition instrument. In either case, it is not necessary tophysically contact the ESD detector of the present invention. Since anESD event would form patterns or switched regions depending on thepolarity and the voltage of the event, a readout unit could be placedadjacent to the detector, allowing each electro-static discharge eventto be recorded. Alternatively, the detector can be observed continuouslyto record the time and conditions of each ESD event. A record of thestatic events could then be linked to a computer for statistical processcontrol.

After the detection of an ESD event, the detector of the presentinvention would need to be reset, i.e., the active region would have tobe returned to its initial condition. This can be accomplished with anexternally packaged reset instrument which uses external magnetic fieldsinduced by an electro-magnetic coil or permanent magnet to “reswitch”the magneto-optic pixels of the detector. The operation of the resetfunction is determined by the same magneto-optic effect that determinesthe sensitivity of the detector to an ESD event. An electro-magnet ofsufficient field strength and uniformity can be used to reset the deviceby subjecting the detector to a short pulse which resets the magneticstate of each magneto-optic pixel.

The present invention is also directed to a method for detecting theoccurrence of electrostatic events utilizing a magneto-optic detector asthe ones herein described. Other methods of the present invention aredirected to detecting the polarity and the magnitude of theelectrostatic discharge event.

The present invention provides a reliable, low cost miniature detectorfor accurately and economically detecting the occurrence, polarity andmagnitude of electrostatic discharge events, including relatively lowlevel events. The present invention is rugged and relativelyinexpensive, allowing the device to be utilized with electroniccomponents and circuit board assemblies to detect ESD events.Additionally, the present invention can also be connected to anelectrostatic discharge sensitive component to prevent the componentfrom being destroyed in the event of a sufficiently high ESD event. Insuch an embodiment, the invention is constructed such that the conductorfor conducting the ESD charge will be destroyed by an ESD event over athreshold chosen to protect the device being maintained fromdestruction. However, the protection mode of the present invention isnot reusable as it acts as a fuse to protect the ESD sensitive devicefrom possible damage. These and other features and advantages of theinvention will become apparent from the following detailed description,when taken in conjunction with the accompanied exemplary drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a magneto-optic array of the present invention;

FIG. 2 is a plan view of a portion of the array of FIG. 1 showing a pairof magneto-optic pixels located on opposites of a conductor; and

FIG. 3 is a plan view of a magneto-optic array showing an array of thepairs of magneto-optic pixels of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention relates to a system for detecting and evaluating theoccurrence, polarity and magnitude of electrostatic discharge (ESD)events. As described herein, reference will be made to numerous specificdetails, such as the composition of magneto-optic devices and the effectof an induced magnetic field on the magnetic and optical properties ofsuch devices, in order to more fully describe the invention. However, itshould be apparent to one skilled in the art that the present inventionis not limited to the exemplary components described, nor their specificarrangements provided for the purposes of illustration.

In the present invention, several embodiments of a novel magneto-opticdevice are described for detecting and evaluating the occurrence of anESD event. These novel devices are also capable of providing additionalinformation not previously available which allows furthercharacterization of the ESD event, thus aiding in the elimination of thecauses of such events from the environment in which the occurrence of anESD event can lead to the destruction or damage of ESD sensitivedevices. Furthermore, if properly configured, the invention is capableof providing protection against an ESD event.

Referring first to FIG. 1, one presently preferred embodiment of anarray of the magneto-optic devices of the present invention is depicted.In this embodiment, a number of magneto-optic devices are arranged in ageometric pattern, or array 2, wherein each magneto-optic device formspixels 4 and 6. These pixels have three stable magnetic states:magnetized up, normal to the surface of the array (the z-direction);magnetized down (also in the z-direction, but into the surface of thearray); and demagnetized, in which there are equal amounts of regionswithin the pixel that are magnetized up or down. FIG. 1 shows pixel 4 asa bright, or unswitched pixel. Pixel 6 shows a darkened pixel which hasbeen switched as a result of an ESD event. The particular manner inwhich the pixels 4 and 6 are affected by an ESD event is described ingreater detail below.

When a conductor, such as a wire, lead or gate carries a current, amagnetic field is induced around the conductor. The strength of themagnetic field will be directly proportional to the level of the currentcarried by the conductor. Where the conductor is in close proximity tothe array 2, this magnetic field, if it reaches a critical level in theregion of the pixel, will cause the pixel to change magnetic states.That is, if the pixels 4 and 6 are originally magnetized in the up ordown states, a sufficient magnetic field induced by the current carriedin the conductor will cause pixels located sufficiently close to theconductor to demagnetize if the strength of the magnetic field exceedsthe critical level.

Demagnetization, or switching magnetic states, of the pixels is a twopart process. First, the magnetic field causes nucleation of a domainwall. The domain wall then propagates throughout the pixel resulting incomplete saturation of the pixel and a change in magnetic state. Amagnetic domain wall is established by nucleation at a nucleation sitewithin the magnetic material of the selected pixel element. This domainwall is formed between the initial condition of magnetization and thenucleated opposite magnet condition. The remainder of the element isswitched by propagating the domain wall through the thickness of thepixel element so that part or all of the entire element exhibits adirection of magnetization opposite to the initial condition.

It is well known in the art that the magnetic field required fornucleation is greater than that required for propagating the domainwall, because domain wall motion is limited by demagnetizing andcoercivity effects. The field established by the selected conductorsdiminishes rapidly as the distance from the conductors increases. Thus,the value or strength of the magnetic field adjacent to the energizedconductors may be quite large, while the field in the region of pixelsfarthest from the energized conductor will be relatively small.

Due to the Faraday effect, the plane of polarization of a beam of lightpassing through a magneto-optic pixel is dependent upon the magneticstate of the pixel. The effect of a magnetic field passing through aconductor upon the magnetic state, and subsequent optical state, of apixel is seen in FIG. 1. In this figure, conductors (not shown) lieadjacent to several rows of switched pixels 8-14. These rows 8-14 depicta row of conductor paths caused by an ESD event. The pixels 4 and 6 ofarray 2 were originally magnetized in a uniform direction. An ESDcurrent was induced in the conductors, resulting in demagnetization ofthe pixels in the vicinity of the conductors so that the magnetic fieldin the region of the pixels exceeded the critical level, initiatingnucleation in the pixels and a change of their magnetic state. FIG. 1depicts the view as seen through a polarizing microscope (not shown) oflight reflected by the pixels 4 and 6 of array 2. In FIG. 1, lightreflected by pixels in their original, magnetized state, as depicted bypixel 4, is rotated by the magnetic state of the pixel, and appearbright under the polarizing microscope. Pixel 6, which is adjacent tothe path of the conductors, have been demagnetized by the ESD inducedcurrent in the conductor, and thus appears darkened when viewed underthe polarizing microscope. The rows of pixels 8-14, likewise, appear asdarkened regions since they too have been switched by the ESD event.

An important advantage of the present invention over prior art ESDsensors is the capability to be reused. As should be apparent, reset ofswitched pixels 6 can be accomplished by subjecting the array 2 to amagnetic field of sufficient strength to switch the magnetic state ofthe demagnetized pixels back to the original state present before theESD event.

The sensitivity of detection of ESD events by the present invention isdependent upon the ESD current levels desired to be detected and thecurrent level desired to be protected against. The minimum detectablecurrent, i.e., the minimum current that will generate a magnetic fieldof sufficient strength to induce a change in the magnetic state of thepixels adjacent to the conductor, is determined by the integrateddistance from all locations of the conductor to the nucleation site ofthe pixel and the angle between the Z-axis of the pixel and a linebetween the center of the conductor and the nucleation site. In otherwords, the minimum detectable current is determined by the z-componentof the magnetic field. The nucleation site of an individual pixel willbe the location in the array where the z-component of the magnetic fieldis a maximum, and is a function of the geometry of the conductor and therelative position of the conductor with respect to the pixel.

The protection level of the present invention is determined by thecurrent-carrying capability of the conductor. In the present invention,the current-carrying capacity of the conductor is a function of theconductor material, the thickness of the conductor, and the width of theconductor. Unlike the detection sensitivity, the protection level isindependent upon the location of the conductor with respect to thepixel.

There are at least five variables available to the designer of a deviceaccording to the present invention that allow the design and manufactureof devices having a wide range of performances to suit a variety ofuses. These variables include: (a) conductor material, (b) width of theconductor (generally limited only to the photolithographic capabilitiesof the fabrication facility), (c) thickness of the conductor (limited bydeposition techniques and photolithographic capabilities of thefacility), (d) the distance from the center of the conductor to thenucleation site (the minimum value of which is the thickness or width ofthe conductor), and (e) magneto-optic material properties.

In its simplest embodiment, the present invention comprises an array ofmagneto-optic pixels 6 centered under a conductor (not shown) lyingalong conductive path 8. The conductor may be insulated from the array 2by an insulating layer deposited during manufacture of the device.Alternatively, the conductor may be either buried in the magneto-opticfilm or lie at the same level as the magneto-optic film forming thepixel 6.

An alternative embodiment of the present invention is depicted in FIG.2. In this embodiment, a pair of magneto-optic pixels 22 and 24 arelocated on each side of a conductor 20. This embodiment of the presentinvention allows the determination of the polarity, or direction ofcurrent flow, of the ESD event. Such a determination may be important inanalyzing the ESD event, as it can indicate on which surface theelectrostatic charge was accumulating. Knowing this fact would aidengineers in eliminating the environmental factors causing theaccumulation of static charge, or in providing suitable groundingconnections for the circuits, circuit assemblies or handlers.

Referring again to FIG. 2, if the pixels 22 and 24 are initiallymagnetized such that the direction of magnetization is in a directioninto the page, a positive current flowing through the conductor 20 inthe direction of arrow 28 will result in a magnetic field around theconductor 20 whose magnetic lines of force are directed into the page inthe area of pixel 24, as depicted by the Xs 28 within pixel 24 and outof the page in the area of pixel 22, as depicted by the Os 26 withinpixel 26. As described previously, the original magnetic state of pixel24 is magnetized in the direction out of the page; thus application ofthe magnetic field resulting from current flow in conductor 20 in thedirection of arrow 29 does not induce a change in the magnetic state ofpixel 24. The direction of the magnetic field in the area of pixel 22,however, is into the page, or opposite the original magnetic state ofpixel 22. If the strength of the magnetic field in the area of pixel 22exceeds the critical value required to induce nucleation in pixel 22,the magnetic state of pixel 22 will change. This change in state will bereadily apparent when pixels 24 and 22 are viewed under a polarizingmicroscope.

As will be apparent to one skilled in the art, if the direction ofcurrent flow in conductor 20 is in the opposite direction to thatindicated by arrow 29, application of the magnetic field resulting fromcurrent flow in conductor 20 in the opposite direction of arrow 29 willalso result in the opposite case to that described above. The directionof the magnetic lines of force will be into the page in the area ofpixel 22, and out of the page in the area of pixel 24. If the strengthof the magnetic field in the area of pixel 24 exceeds the critical valuerequired to induce nucleation in pixel 24, the magnetic state of pixel24 will change.

While the embodiment of the present invention depicted in FIG. 2 isuseful in determining the polarity of an ESD event, the event will onlybe detected if the current flowing through the conductor 20 issufficient to generate a magnetic field with a strength exceeding thecritical threshold required to cause a change in the magnetic state ofpixels 22 and 24. Thus, such a detector indicates that an ESD eventhaving some minimum magnitude has occurred, but provides no informationregarding how large the ESD actually was.

A further embodiment of the present invention that is capable ofproviding information about the magnitude of the ESD event, beyond aminimum magnitude, is depicted in FIG. 3. In this embodiment, thedetector comprises a set of pixels, such as pixels 32-40, that arearranged in an array next to conductor 30. As depicted, each pixel isseparated by different distances than any other pixel. Since thestrength of the magnetic field which surrounds the conductor 30, causedby the individual current flowing through the conductor 30, is inverselyproportional to the distance from the conductor 30 at the point themagnetic field strength is measured, different pixels will have alteredmagnetic states dependent on the strength of the magnetic field, whichis related to the magnitude of current flowing through the conductor 30induced by the ESD event. Pixels 42-50 can be added on the opposite sideof conductor 30 to provide capability of determining polarity.

For example, if it is desired to know the voltage and polarity of theESD event to within 50 volts (V) for a particular line resistance, thefirst pair of pixels 32 and 42 would be separated by a distancesufficient to ensure that the magnetic field generated by a 50 V currentflowing through the conductor 30 would be sufficient strong to induce achange in the magnetic state of one of pixels 32 and 42. The second pairof pixels 34 and 44 would be separated by a distance that would requirea magnetic field strength resulting from a 100 V current flowing throughthe conductor 30 before the pixels 34 and 44 would change magneticstate. The number of pairs of pixels is limited only by the requirementsof the designer, that is, by the maximum voltage ESD event that isdesired to be detected. It should be noted that the geometry depicted inFIG. 3 is for illustrative purposes only; the location of the nucleationsites in the various pixels is dependent upon the geometry of the arrayand the location of the conductor 30. These factors are easily accountedfor in the design of the device, and will be apparent to one skilled inthe art.

An alternative approach to providing a detector that is capable ofdetermining the magnitude of an ESD event would be to use theconfiguration depicted in FIG. 2, but connecting a number of pairs ofpixels in parallel using. conductors 20 having different lineresistances in each pair of pixels. For example, if the line resistancesof the conductor 20 of two pixel pairs were 500 and 1000 ohms, an ESDevent of 100 V would cause currents of 200 ma and 400 ma to flow in theconductor 20 respectively. Since the strength of the magnetic field isproportional to the magnitude of the current flowing through theconductor 20, a pixel pair that requires a magnetic field strengthgreater than can be generated by a current of 200 ma will not changestates during the 100 V ESD event.

Typically, the present invention, including, but not limited to thevarious embodiments described above, are fabricated from a magneto-opticfilm which is grown using well known fabrication techniques on anon-magnetic substrate. The fabrication process will vary depending uponthe geometry of the devices, and particularly depending on the locationof the conductors in relation to the pixels. For example, a differentprocess may be required depending on whether the detector is designedwith the conductor passing over the top of the magneto-optic region, orwhether the conductor passes next to the magneto-optic region.

In fabricating a detector where the conductor passes over themagneto-optic film, a magneto-optic epitaxial film is deposited on awafer using techniques well known in the semiconductor industry.Alignment marks for aligning masking levels in subsequent fabricationsteps are then photolithographically defined and machined into themagneto-optic film using an ion beam. The wafer is photolithographicallydefined and masked using a standard photoresist to accept an ionimplanted dopant, such as boron, into the regions of the wafer wherebubble nucleation and/or collapse is desired, reducing the magneticfields necessary for inducing a change of magnetic state in the desiredmagneto-optic regions by approximately an order of magnitude.

The magneto-optic layer is segmented by laying out a desiredsegmentation pattern using a photoresist etch mask, followed by ion beammilling or chemical etching. An insulator material is then deposited onthe surface of the magneto-optic layer to provide proper separationbetween the conductor and the nucleation site in the magneto-opticlayer. The wafer may then be metallized and patterned to produce thedesired electrode patterns for ESD detection and protection and forsubsequent electrical interconnection. The metallized pattern also actsas a reflector for the backside illumination and reading of themagnetization state of the magneto-optic pixels. A similar fabricationprocess is used to fabricate detectors with the conductor located nextto the magneto-optic pixels, with the exception that no insulation layeris necessary between the conductor and the magneto-optic layer. Suitablematerials for the conductor include aluminum, aluminum alloys andpalladium.

For example, the device can be fabricated on an epitaxial film grown ona Czochralski non-magnetic garnet substrate. The single crystal boule issliced and polished into high optical quality 0.25 mm thick substratewafers. These substrates are transparent, non-magnetic single crystalsand are basically Gd₃Ga₅O₁₂ doped with Ca, Mg, and Zr to increase thesize of the crystal lattice so that the crystal can accommodate a largeamount of bismuth in the magnetic iron garnet film which is grownepitaxially on the substrate. The bismuth enhances the Faraday rotationof the film.

The packaging of the device is determined by its desired final use, thatis, as a permanent component in an electronic circuit or as a temporarymonitor during certain fabrication/testing steps. The device may bepassivated to protect against environmental degradation of the conductorand isolation resistances using typical well known passivationtechniques. Electrical connection to the device could be made by any ofthe methods well known and commonly used in the semiconductor industrysuch as wire bonding or bump bonding. Connection pads may be exposedthrough the passivation layer, and additional metal may be deposited ifneeded to facilitate interconnection. The wafer may be diced, cleanedand inspected using standard semiconductor techniques, and theindividual die may then be attached to a suitable substrate, oralternatively, directly to a circuit using standard hybridizationtechniques.

In practice, the device may be packaged in a TO-18 or equivalent packagefor leaded insert into an appropriately fabricated circuit board. Thelid would have a transparent window to allow the magnetic state of thedetector to be read by an analyzer. The device can be manufactured indimensions as small as about 0.5 mm by 0.5 mm, with a thickness as smallas about 0.3 mm.

The device may also be packaged in a surface mount package for insertiononto hybrids, multi-chip modules or surface mounted circuit boards. Thisconfiguration may also require a window to allow the magnetic state ofthe detector to be read. Another alternative is to package the detectorin a plastic clip lead package which provides a low cost hermeticpackage for external monitoring that would not require a window to allowthe magnetic state of the detector to be read by an analyzer.

ESD events that are monitored by the detector will cause patterns ofpixels to change their magnetic states in response to the magneticfields generated within the conductors by the ESD event. Reading themagnetic state of the pixels of the detector may be accomplished byobserving light reflected by the pixel through a polarizing microscope,as described previously. Alternatively, a pattern recognition unit maybe provided to automatically scan the pixels and determine theirmagnetic states, and record the patterns for analysis. If desired, thedetectors may be fashioned to allow for continuous monitoring. Thepattern detector or other automatic magnetic state reader may also beconnected to a computer. Such a connection would allow the informationprovided by the detector to be used for statistical process control.

After the detection of an ESD event, and subsequent reading of thedetector, either manually or automatically, the pixels of the detectormay be reset. This resetting of the pixels requires that the pixel arraybe subjected to a uniform magnetic field of sufficient strength toinduce a change in the magnetic state of those pixels that were switchedby the ESD event. The generation of this magnetic field may beaccomplished using an externally packaged reset unit, which may bebattery powered if portability is required. Alternatively, the detectormay be packaged with an on-board coil to generate the magnetic field,thus eliminating the need for an external reset package.

From the above, it may be seen that the present invention provides asystem and method of monitoring ESD events which is both sophisticatedand reusable. It also allows for remote and real time readout, as wellas diagnostic measurement. Finally, the invention can be made to besacrificial in the event of an ESD event which would likely damage thedevices being monitored.

While several forms of the invention have been illustrated anddescribed, it will also be apparent that various modifications can bemade without departing from the spirit and scope of the invention.Accordingly, it is not intended that the invention be limited, except bythe appended claims.

We claim:
 1. An electrostatic discharge event detector comprising: asubstrate; at least one conductor patterned from a metal layer depositedon the substrate, the conductor acting as an antenna for receiving avoltage produced by an electrostatic discharge, the received voltageinducing a current to flow through the conductor sufficient to generatea magnetic field having a field polarity around the conductor; a firstmagneto-optic element having a first magnetic state having a firstpolarity; a second magneto-optic element having a first magnetic statehaving a second polarity, the second polarity being opposite from thefirst polarity of the first magnetic state of the first magneto-opticelement, the first and second magneto-optic elements being formed from afilm deposited on the substrate and patterned such that the first andsecond magneto-optic elements are formed on opposite sides of theconductor, the first and second magneto-optic elements being capable ofswitching from the first magnetic state to a second magnetic having apolarity opposite of the polarity of the first magnetic state inresponse to a magnetic field having a polarity opposite of the polarityof the first magnetic state and having a field strength that exceeds apredetermined field strength at the location of the magneto-opticelement.
 2. The detector claim 1 wherein the first and secondmagneto-optic elements form a first pair of magneto-optic pixels, andfurther comprising at least one additional pair of magneto-optic pixelslocated on opposite sides of the conductor.
 3. An electrostaticdischarge event detector for detecting the occurrance and polarity of anelectrostatic event comprising: a substrate; at least one conductorformed on the substrate, the conductor forming an antenna capable ofreceiving a voltage induced by an electro-static event having apolarity, the received voltage causing a current to flow in theconductor in a direction related to the polarity of the electrostaticevent, the current flow in the conductor causing a magnetic field to beformed about the conductor, the magnetic field having a field polarityrelated to the polarity of the electro-static event; a set ofmagneto-optic elements formed on the substrate and located adjacent tothe conductor, each magneto-optic element having a first magnetic statehaving a first polarity and being capable of switching to a secondmagnetic state having a second polarity different from the firstpolarity in response to the field polarity of the magnetic field formedabout the conductor when the magnetic field has a field strength thatexceeds a predetermined field strength at the location of themagneto-optic element and a field polarity opposite to the firstpolarity of the magneto-optic element; a second set of magneto-opticalelements formed on the substrate and located adjacent to the conductoron a side opposite the first set of magneto-optical elements, eachmagneto-optic element of the second set having a first magnetic statehaving a first polarity and being capable of switching to a secondmagnetic state having a second polarity different from the firstpolarity in response to the field polarity of the magnetic field formedabout the conductor when the magnetic field has a field strength thatexceeds a predetermined field strength at the location of themagneto-optic element and a field polarity opposite to the firstpolarity of the magneto-optic element.
 4. The detector of claim 3,wherein the first magnetic state of the magneto-optic elements has afirst direction and the second magnetic state of the magneto-opticelements has a second direction and the magnetic field around theconductor has a direction dependent on the polarity of the electrostaticevent, and wherein the magneto-optic elements change from the firstmagnetic state to the second magnetic state in response to the magneticfield when the direction of the magnetic field is different from thefirst direction of the first magnetic state of the magneto-opticelements.
 5. A method for determining the polarity of an electrostaticdischarge comprising the steps of: providing an electrostatic dischargeevent detector having a conductor and a pair of magneto-optic elementslocated on opposite sides of the conductor, the elements having a firstmagnetic state and a second magnetic state and capable of switchingbetween the first and second magnetic states in response to anelectro-magnetic field generated in the conductor by an electrostaticdischarge; detecting an electrostatic discharge in a general area of thelocation of the detector by sensing when a current is induced in theconductor by the electrostatic discharge, the induced current generatingthe electro-magnetic field around the conductor, the generatedelectromagnetic field having a polarity and a field strengthrepresentative of the polarity and magnitude of the electrostaticdischarge; determining the magnetic state of the magneto-optic elementsby illuminating the magneto-optic elements with polarized light andobserving the light from the elements, wherein the polarity of theelectrostatic discharge is determined by observing which one of theelements has the second magnetic state and which element has the firstmagnetic state.